Electroluminescent signs such as neon, traditional fluorescent backlit signs and channel lettering typically direct light into large angles in the horizontal and vertical directions. In many cases, the light does not need to be directed in to certain angular directions because the sign is not normally viewed from that direction. For example, most signs are typically viewed in the horizontal direction with only a slight vertical downward direction needed. The light that is sent high in the vertical direction upward is often wasted and causes significant light pollution. This also increases the running costs of the sign due to the increased electrical power and brighter bulb requirements.
Fluorescent light bulb-based electroluminescent signs are generally rectangular in shape due to the typical long cylindrical shape of the fluorescent bulbs. These signs typically use white reflecting light boxes and symmetrically scattering diffusers to spread the light in all directions. Channel letters often diffuse light into wide angles in the horizontal and vertical directions using symmetric light scattering films or plastic. A significant amount of light is absorbed or directed into unnecessary directions when using these designs.
Improvements in solid state light sources such as light emitting diodes (LEDs) are continuously increasing their efficacy with their luminous output per electrical watt approaching fluorescent sources. LEDs are essentially point light sources, as opposed to the extended light sources of fluorescent bulbs. Thus, the light from an LED can be controlled more effectively (even when multiple LEDs are used) with the proper optical films and materials. Traditional diffusers used with point sources such as LEDs can exhibit speckle. This is typically avoided by using one or more symmetric diffusers, thick diffusing films, white scattering surfaces such as printed white dots or white light box walls. However, these methods inefficiently scatter the light into large angles and undesirable directions.
Current optical films used with electroluminescent signs include symmetrically diffusing films and prismatic films. For example, the 3M Diffuser Films 3635-30 and 3635-70 are reflecting and transmitting diffuser films that transmit 30% and 70% of the incident light, respectively. The light is diffused symmetrically forward (transmitting) or backward (reflecting) through scattering from particles. While some light boxes require a significant amount of the light to be reflected back toward the white light box, these designs scatter light inefficiently into regions where the light is absorbed. The components of the light box, including the white walls and films, absorb a significant amount of the light that is scattered throughout the light box multiple times. Alternatively, more efficient and thinner designs such as edge-lit or waveguide-based electroluminescent signs are sometimes used. Often with edge-lit or waveguide designs, white scattering dots are printed on a waveguide or film coupled to a waveguide. These dots scatter the light symmetrically, and much of the light is scattered into directions where it is not needed (such as the vertical direction in many applications). This results in an inefficient electroluminescent sign.
Prismatic films such as 3M's Optical Lighting Film (OLF) and 3M's Brightness Enhancement Film (BEF) are sometimes used to direct the light in the large angles in one plane back toward the forward direction. For example, the light from a fluorescent bulb-based edge-lit sign has printed dots on the back that scatter the light in symmetrical directions. A diffuser is often added to the top of the waveguide to blend the non-uniformities of the white dots together and scatter the light into larger angles in both directions. BEF film can be placed with the prisms aligned in the horizontal direction to direct some of the light in the large angles in the vertical directions toward the forward direction. The first area of inefficiency is the white dots that scatter a portion of the light back toward the fluorescent bulb, where it leaves the waveguide and is absorbed by the bulb or other components. Secondly, the symmetrically-scattering diffuser scatters the light into large angles where it is unused and also scatters more light backward towards the waveguide where some of it is absorbed. The BEF directs some of the light in one plane, but not all of the light, towards a more forward direction. All of these components and films add to the volume and cost of the sign with the BEF film being expensive to manufacture due to microreplication techniques.